1. Field of the Invention
This invention relates to semiconductor light emitting diodes (LEDs) and more particularly to LED packages and LED packaging methods.
2. Description of the Related Art
Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted omnidirectionally from the active layer and from all surfaces of the LED. The useful light is generally emitted in the direction of the LED's top surface, which is usually p-type.
Conventional LEDs cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, the light from red, green and blue LEDs, or blue and yellow LEDs can be combined to produce white light. In addition, different colors of light are often generated from different types of LEDs which can require complex fabrication to combine in one device. The resulting devices can also require complicated control electronics since the different diode types can require different control voltages. Long term wavelength and stability of these devices is also degraded by the different aging behavior of the different LEDs.
More recently, the light from a single blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc., which comprise blue LEDs surrounded by a yellow phosphor powder. see also U.S. Pat. No. 5,959,316 to Lowery, entitled Multiple Encapsulation of Phosphor-LED Devices.] The surrounding material “downconverts” the wavelength of some of the LED light, changing its color. For example, if a nitride based blue emitting LED is surrounded by a yellow phosphor, some of the blue light will pass through the phosphor without being changed while the remaining light will be downconverted to yellow. The LED will emit both blue and yellow light, which combine to produce white light. Other examples of LEDs using this approach include U.S. Pat. No. 5,813,753 to Vriens et al.
A common type of LED packaging is known as a “glob-in-a-cup” method, where an LED resides at the bottom of a cup-like recession. A phosphor containing matrix material (e.g. phosphor powder distributed in an encapsulant such as silicone or epoxy) is injected into and fills the cup, surrounding and encapsulating the LED. The matrix material is then cured to harden the encapsulant around the LED. This packaging, however, can result in an LED package having significant variation of the color and hue of emitted light at different viewing angles with respect to the package. This color variation can be particularly pronounced in packages where the phosphor containing matrix material extends above the “rim” of the cup in which the LED resides, resulting in a predominance of converted light emitted sideways into high viewing angles (e.g. at 90 degrees from the optic axis). This problem can be made worse by the limited amount of unconverted LED emitting at high viewing angles. The unconverted LED light is typically reflected away from at high viewing angles by the side walls of the cup so that little corresponding unconverted light emits at these angles.
Another method for packaging LEDs comprises direct coupling of phosphor particles onto the surfaces of the LED. This “white chip” method can result in significant improvement of the color uniformity as a function of viewing angle with one reason for this improvement being the source of the converted light and unconverted light being at close to the same point in space. For example, a blue emitting LED covered by a yellow converting material can provide a substantially uniform white light source because the converting material and LED are at close to the same point in space. This method, however, typically requires complex and expensive processing, such as electrophoretic deposition, to achieve uniform phosphor coating directly on an LED.